The present invention relates to a system for assisting the regeneration of depollution means associated with oxidation catalyst-forming means and integrated in an exhaust line of a motor vehicle diesel engine.
More particularly, the invention relates to a system in which the engine is associated with a common manifold or “rail”, for feeding fuel to the cylinders of the engine and adapted to implement, at constant torque, a regeneration strategy by injecting fuel into the cylinders in at least one postinjection operation.
During regeneration of depollution means such as a particle filter, for example, stages in which the vehicle accelerator pedal is being raised (no fuel injection in normal operation), and stages in which the engine is idling (very low exhaust temperature), a problematic since, they cause the exhaust temperature to drop, i.e. the temperature of the exhaust line and of elements integrated therein.
The use of one or more postinjections during these stages in the lifetime of the engine serves to limit the temperature drop in the exhaust line, by relying on the catalytic conversion of the HCs produced by combustion of the or each postinjection in the engine.
However, such strategies rely on an exothermic reaction being produced by the catalyst-forming means, which means, e.g. comprising an oxidation catalyst for a NOx trap with a CO/HC oxidation function, are assumed to be active.
During stages in which the engine is returning to idling, as a result of the accelerator pedal being raised, there is no main injection nor any pilot injection, and as a result, the or each postinjection does not burn in the cylinder, which does no more than vaporize the fuel in the form of HCs that are subsequently converted by the catalyst-forming means.
The temperature at the inlet to the oxidation catalyst-forming means is thus very low, and in spite of the exothermic reaction produced by the catalytic combustion of the HCs coming from the or each postinjection, the front face of the catalyst-forming means cools down progressively and its conversion activity becomes progressively deactivated.
During a prolonged stage of returning to idling, it can happen then the catalyst-forming means are therefore not sufficiently active to convert all of the HCs, which leads to HC peaks downstream front the catalyst-forming means, or even the blue smoke and/or exhaust odors.
Furthermore, the use of postinjections leads to the lubricating oil being diluted by fuel, thereby degrading its lubrication properties, and in particular lowering its viscosity, and if the viscosity becomes too low, that can lead to damage to the engine.